An ion source is a device that ionizes gas molecules and then focuses, accelerates, and emits them as a narrow beam. This beam is then used for various technical and technological purposes such as cleaning, activation, polishing, thin-film coating, or etching.
An example of an ion source is the so-called Kaufman ion source, also known as a Kaufman ion engine or an electron-bombardment ion source described in U.S. Pat. No. 4,684,848 issued to H. R. Kaufman in 1987.
This ion source consists of a discharge chamber, in which plasma is formed, and an ion-optical system, which generates and accelerates an ion beam to an appropriate level of energy. A working medium is supplied to the discharge chamber, which contains a hot cathode that functions as a source of electrons and is used for firing and maintaining a gas discharge. The plasma, which is formed in the discharge chamber, acts as an emitter of ions and creates, in the vicinity of the ion-optical system, an ion-emitting surface. As a result, the ion-optical system extracts ions from the aforementioned ion-emitting surface, accelerates them to a required energy level, and forms an ion beam of a required configuration. Typically, aforementioned ion sources utilize two-grid or three-grid ion-optical systems.
A disadvantage of such a device is that it requires the use of ion accelerating grids and that it produces an ion beam of low intensity.
Attempts have been made to provide ion sources with ion beams of higher intensity by holding the electrons in a closed space between a cathode and an anode where the electrons could be held. For example, U.S. Pat. No. 4,122,347 issued in 1978 to Kovalsky, et al. describes an ion source with a closed-loop trajectory of electrons for ion-beam etching and deposition of thin films, wherein the ions are taken from the boundaries of a plasma formed in a gas-discharge chamber with a hot cathode. The ion beam is intensified by a flow of electrons which are held in crossed electrical and magnetic fields within the accelerating space and which compensate for the positive spatial charge of the ion beam.
A disadvantage of the devices of such type is that they do not allow formation of ion beams of chemically-active substances when these ion beams are used for treating large surface areas. Other disadvantages of the aforementioned devices are short service life and high non-uniformity of ion beams.
U.S. Pat. No. 4,710,283 issued in 1997 to Singh, et al. describes a cold-cathode type ion source with crossed electric and magnetic fields for ionization of a working substance wherein entrapment of electrons and generation of the ion beam are performed with the use of a grid-like electrode. This source is advantageous in that it forms belt-like and tubular ion beams emitted in one or two opposite directions.
The ion source with a grid-like electrode of the type disclosed in U.S. Pat. No. 4,710,283 also has a number of disadvantages consisting in that the grid-like electrode makes it difficult to produce an extended ion beam and in that the ion beam is additionally contaminated as a result of sputtering of the material from the surface of the grid-like electrode. Furthermore, with the lapse of time the grid-like electrode is deformed whereby the service life of the ion source as a whole is shortened.
Other publications (e.g., Kaufman H. R., et al. (End Hall Ion Source, J. Vac. Sci. Technol., Vol. 5, Jul./Aug., 1987, pp. 2081-2084; Wyckoff C. A., et al., 50-cm Linear Gridless Source, Eighth International Vacuum Web Coating Conference, Nov. 6-8, 1994)) disclose an ion source that forms conical or belt-like ion beams in crossed electrical and magnetic fields. The device consists of a cathode, a hollow anode with a conical opening, a system for the supply of a working gas, a magnetic system, a source of electric supply, and a source of electrons with a hot cathode. A disadvantage of this device is that it requires the use of a source of electrons with a hot or hollow cathode and that it has electrons of low energy level in the zone of ionization of the working substance. These features create limitations for using chemically-active working substances. Furthermore, a ratio of the ion-emitting slit width to a cathode-anode distance is significantly greater than 1, and this decreases the energy of electrons in the charge space, and hence, hinders ionization of the working substance. Configuration of the electrodes used in the ion beam of such sources leads to a significant divergence of the ion beam. As a result, the electron beam cannot be delivered to a distant object and is to a greater degree subject to contamination with the material of the electrode. In other words, the device described in the aforementioned literature is extremely limited in its capacity to create an extended uniform belt-like ion beam. For example, at a distance of 36 cm from the point of emission, the beam uniformity did not exceed .+-.7%.
Russian Patent No. 2,030,807 issued in 1995 to M. Parfenyonok, et al. describes an ion source that comprises a magnetoconductive housing used as a cathode having an ion-emitting slit, an anode arranged in the housing symmetrically with respect to the emitting slit, a magnetomotance source, a working gas supply system, and a source of electric power supply.
FIGS. 1 and 2 schematically illustrate the aforementioned known ion source with a circular ion-beam emitting slit. More specifically, FIG. 1 is a sectional side view of an ion-beam source with a circular ion-beam emitting slit, and FIG. 2 is a sectional plan view along line II--II of FIG. 1.
The ion source of FIGS. 1 and 2 has a hollow cylindrical housing 40 made of a magnetoconductive material such as Armco steel (a type of a mild steel), which is used as a cathode. Cathode 40 has a cylindrical side wall 42, a closed flat bottom 44 and a flat top side 46 with a circular ion emitting slit 52.
A working gas supply hole 53 is formed in flat bottom 44. Flat top side 46 functions as an accelerating electrode. A magnetic system in the form of a cylindrical permanent magnet 66 with poles N and S of opposite polarity is placed inside the interior of hollow cylindrical housing 40 between bottom 44 and top side 46. An N-pole faces flat top side 46, and S-pole faces bottom side 44 of the ion source. The purpose of magnetic system 66 with a closed magnetic circuit formed by parts 66, 40, 42, and 44 is to induce a magnetic field in ion emitting slit 52. It is understood that this magnetic system is shown only as an example and that it can be formed in a manner described, e.g., in aforementioned U.S. Pat. No. 4,122,347. A circular annular-shaped anode 54, which is connected to a positive pole 56a of an electric power source 56, is arranged in the interior of housing 40 around magnet 66 and concentric thereto. Anode 54 is fixed inside housing 40 by means of a ring 48 made of a non-magnetic dielectric material such as ceramic. Anode 54 has a central opening 55 in which aforementioned permanent magnet 66 is installed with a gap between the outer surface of the magnet and the inner wall of opening 55. A negative pole 56b of the electric power source is connected to housing 40, which is grounded at GR.
Located above housing 40 of the ion source of FIGS. 1 and 2 is a sealed vacuum chamber 57 which has an evacuation port 59 connected to a source of vacuum (not shown). An object OB to be treated is supported within chamber 57 above ion emitting slit 52, e.g., by gluing it to an insulator block 61 rigidly attached to the housing of vacuum chamber 57 by a bolt 63 but so that object OB remains electrically and magnetically isolated from the housing of vacuum chamber 57. However, object OB is electrically connected via a line 56c to negative pole 56b of power source 56. Since the interior of housing 40 communicates with the interior of vacuum chamber 57, all lines that electrically connect power source 56 with anode 54 and object OB should pass into the interior of housing 40 and vacuum chamber 57 via conventional commercially-produced electrical feedthrough devices which allow electrical connections with parts and mechanisms of sealed chambers without violation of their sealing conditions. In FIG. 1, these feedthrough devices are shown schematically and designated by reference numerals 40a and 57a. Reference numeral 57b designates a seal for sealing connection of vacuum chamber 57 to housing 40.
The known ion source of the type shown in FIGS. 1 and 2 is intended for the formation of a unilaterally directed tubular ion beam. The source of FIGS. 1 and 2 forms a tubular ion beam IB emitted in the direction of arrow A and operates as follows.
Vacuum chamber 57 is evacuated, and a working gas is fed into the interior of housing 40 of the ion source. A magnetic field is generated by magnet 66 in an ion-accelerating space 52a between anode 54 and cathode 40, whereby electrons begin to drift in a closed path within the crossed electrical and magnetic fields. A plasma 58 is formed between anode 54 and cathode 40. When the working gas is passed through the ionization space, tubular ion beam IB, which propagates in the axial direction of the ion source shown by an arrow A, is formed in the area of ion-emitting slit 52 and in ion-accelerating space 52a between anode 54 and cathode 40.
The above description of the electron drift is simplified to ease understanding of the principle of the invention. In reality, the phenomenon of generation of ions in the ion source with a closed-loop drift of electrons in crossed electric and magnetic fields is of a more complicated nature and consists in the following.
When, at starting the ion source, a voltage between anode 54 and cathode 40 reaches a predetermined level, a gas discharge occurs in anode-cathode space 52a. Inside the ion-emitting slit, the crossed electric and magnetic fields force the electrons to move along closed cycloid trajectories. This phenomenon is known as "magnetization" of electrons. The magnetized electrons remain drifting in a closed space between two parts of the cathode, i.e., between those facing parts of cathode 40 which form ion-emitting slit 52. The radius of the cycloids is, in fact, the so-called doubled Larmor radius R.sub.L which is represented by the following formula: ##EQU1## where m is a mass of the electron, B is the strength of the magnetic field inside the slit, V is a velocity of the electrons in the direction perpendicular to the direction of the magnetic field, and lel is the charge of the electron. In electromagnetism, the Larmor radius is known as the radius along which a charged particle moves in a uniform magnetic field, which causes its travel in a circular path in a plane perpendicular to the magnetic field.
It is required that the height of the electron drifting space in the ion-emission direction be much greater than the aforementioned Larmor radius. This means that a part of the ionization area penetrates ion-emitting slit 52 where electrons can be maintained in a drifting state over a long period of time. In other words, a spatial charge of high density is formed in ion-emitting slit 52.
When a working medium, such as argon which has neutral molecules, is injected into the slit, the molecules are ionized by the electrons present in this slit and are accelerated by the electric field. As a result, the thus formed ions are emitted from the slit towards the object. Since the spatial charge has high density, an ion beam of high density is formed.
Thus, the electrons do not drift in a plane, but rather along cycloid trajectories across ion-emitting slit 52. However, for the sake of convenience of description, here and hereinafter and in the claims, the term "electron drifting plane" will be used.
The diameter of the tubular ion beam formed by means of such an ion source may reach 500 mm and more.
The ion source of the type shown in FIG. 1 is not limited to a cylindrical configuration and may have an elliptical or an oval-shaped cross section as shown in FIG. 3. In FIG. 3 the parts of the ion beam source that correspond to similar parts of the previous embodiment are designated by the same reference numerals with an addition of subscript OV. Structurally, this ion source is the same as the one shown in FIG. 1 with the exception that a cathode 40.sub.OV, anode 54.sub.OV, a magnet 66.sub.OV, and hence an emitting slit (not shown in FIG. 3), have an oval-shaped configuration. As a result, a belt-like ion beam having a width of up to 1400 mm can be formed. Such an ion beam source is suitable for treating large-surface objects when these objects are passed over ion beam IB emitted through emitting slit 52.
With 1 to 3 kV voltage on the anode and various working gases, this source makes it possible to obtain ion beams with currents of 0.5 to 1A. In this case, an average ion energy is within 400 to 1500 eV, and a nonuniformity of treatment over the entire width of a 1400 mm-wide object does not exceed .+-.5%.
A disadvantage of the aforementioned ion source with a closed-loop ion-emitting slit is that the position of the tubular ion beam emitted from this source remains unchanged with respect to the surface of object OB being treated. However, the aforementioned tubular beam has a non-uniform distribution of the ion beam current in the cross-section of the beam and hence on the surface of the object OB. More specifically, the ion current density across the beam is greater in the center of the beam and is smaller on the edges of the beam.
Pending U.S. patent application Ser. No. 09/161,581 filed by the same Applicants on Sep. 28, 1998 discloses a closed-loop slit cold-cathode ion source where uniformity of treatment of an object is achieved by shifting either an object with respect to a stationary ion beam or by shifting the anode with respect to cathode or vice verse. Such displacements cause variations in relative positions between the object and the beam whereby even with some non-uniformity in the ion current density distribution in the beam, the surface of the object is treated with an improved uniformity.
A disadvantage of such a device is that the ion source or the ion-beam sputtering system should have movable parts which makes the construction of such source or system more complicated and expensive.